1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs a folding operation such as a Z-shape folding operation on a sheet ejected from an image forming apparatus like a printing apparatus. In addition, the present invention relates to the image forming apparatus equipped with the sheet processing apparatus.
2. Description of the Related Art
Up until now, a known sheet processing apparatus is included in an image forming apparatus such as a copier and a printer and performs a folding operation such as a Z-shape folding operation on a sheet on which an image has been formed by an image forming unit and the like. In such a sheet processing apparatus, a member with which the tip end or the rear end of a sheet comes into contact is provided, for example, on a conveyance path for conveying the sheet on which an image has been formed. When the sheet comes into contact with the member, a deflection is formed at the middle part of the sheet. Then, the middle part of the sheet is conveyed between rollers serving as folding units, so that folding lines are formed on the sheet.
Further, for example, Patent Document 1 discloses another method for forming folding lines on a sheet. In other words, there is provided a space in which the sheet is deflected so as to form a loop on a conveyance path, and a pressing claw is put in the loop to make the tip end of the loop mesh with a folding unit. Thus, folding lines are formed on the sheet.
However, the above methods have following drawbacks. Specifically, if there is a large distance between conveyance guide plates constituting the conveyance path provided between the folding unit and the contact member with which the tip end or the rear end of the sheet comes into contact, the deflection formed at the middle part of the sheet has a high degree of freedom. As a result, the deflection formed at the middle part of the sheet fluctuates. Thus, due to the fluctuation of the deflection formed at the middle part of the sheet, the position of the middle part of the sheet, on which folding lines have been formed by the folding unit, is shifted from an expected folding position. As a result, the folding accuracy of the sheet also fluctuates.
Further, when a folding operation is performed on the same sheet plural times, a part on which folding lines have been formed on an upstream side comes into contact with the contact member as the tip end part or the rear end part of the sheet so that the sheet has a deflection at its middle part. However, if the distance between the conveyance guide plates is large as described above, an unnecessary deflection is formed at the middle part of the sheet. As a result, the position of the folding lines formed on the upstream side is shifted from the folding position of the sheet conveyed between the folding units, which results in the occurrence of a so-called box folding phenomenon.
Patent Document 1: JP-A-2006-62810